Conventionally, there is known a buckle with a hook elastically engaging with engagement grooves provided on a band (or a belt), in which the locked state can be released at any time.
In Japanese Patent No. 4368023 (Patent Document 1), the inventors propose, as a band with a buckle having a surface as flat as possible by thinning the thickness of the buckle, a band with a buckle connected annularly while one end of the band is inserted to a tubular buckle provided on the other end of the band to adjust the circumferential length of the band, in which a saw-toothed engagement recessed part is formed along an edge of the one end, and an engagement projecting part is provided in an insertion space of the buckle, so that the one end of the band can freely move forward through the insertion space but cannot retreat, and a guide for restricting swing of the band within a plane surface orthogonal to the forward direction of the band is provided at least at a position close to the engagement projecting part of the insertion space.
The above-described band with a buckle is adequate as a band for wearing a precision device such as a wristwatch around an arm because the buckle thickness is extremely thin. However, the engagement projecting part is elastically engaged with the engagement recessed part, similarly to the conventional buckle, which has caused a problem that the elasticity is deteriorated with time and the engagement strength is reduced.
Moreover, in Japanese Patent No. 5049399 (Patent Document 2), the inventors propose, as an invention related to a synthetic resin buckle particularly for a product requiring comparatively strong fastening force such as an athletic shoe, sports shoe, a snowboard boot, and a protector locking belt, a buckle allowing a band to freely approach the buckle but not allowing the band to retreat in a locked state where an engagement pawl of the buckle is engaged with a saw-toothed engagement groove formed on the outer surface of the band, the buckle including a buckle body that has a bottom plate and a top frame facing each other, and both side plates connecting the bottom plate and the top frame, with the interior of the buckle body serving as an insertion space for the band, and an operation plate horizontally installed inside the top frame and having, at the lower surface of one end, the engagement pawl that can be engaged with the engagement groove of the band, while forming the other end as a pressing part for releasing the engagement, the operation plate being supported by a connection shaft for connecting both side plates at a midpoint between the engagement pawl and the pressing part such that the operation plate is pivotable around the midpoint serving as a fulcrum, in which a sliding piece horizontally movable on the upper part of the operating plate is installed at the buckle body, and pivot of the operation plate is restricted in a state where the sliding piece has moved to one end side of the operation plate, while the lower surface of the sliding piece is in contact with an upper surface of the pressing part in a state where the sliding piece has moved to the pressing part side.
This buckle has a locking mechanism, thus preventing the release of engagement even against pressing force acting directly on the pressing part. However, the engagement pawl is elastically engaged with the engagement groove, similarly to the conventional buckle, which has caused a problem that, when the released state continues for a long time by the action of the sliding piece, the elasticity is deteriorated with time due to a creep phenomenon (characteristics in which synthetic resin is deformed slowly when strong pressure is applied thereon for a long time) and the engagement strength between the band and the buckle is reduced. Moreover, when the strength of the connection shaft is increased using hard material such as metal or resin not having elasticity, it is necessary to provide a separate component such as a coil spring. When such a separate component is provided, the buckle structure becomes complicated, and thus it has been difficult to produce a buckle that is light, small, and easy to produce.
Patent Document 1: Japanese Patent No. 4368023
Patent Document 2: Japanese Patent No. 5049399